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The window coating (close-up of molecules) was shown to enhance heat transfer from sunlight outdoors (orange arrows) into a building interior while still allowing for ample light transmission (yellow arrow). Credit: image provided by Julian Wang. All Rights Reserved.

By Gabrielle Stewart

Double-pane windows, which sandwich a layer of insulating air, can offer improved energy efficiency over single-pane windows, but replacing existing single-pane windows can be expensive or technically challenging. A more economical but less effective option is to coat single-pane windows with translucent metallic films capable of absorbing some solar heat in winter without compromising clarity of the glass. To improve the coating’s efficacy, Penn State researchers say that nanoscale technology may help elevate thermal performance to that of double-pane windows in winter. 

A team in the Penn State Department of Architectural Engineering examined the energy-saving properties of a coating comprising nanoscale components that can reduce heat loss and better absorb heat. They also completed the first comprehensive energy-savings analysis of the material at the building scale. The researchers published their findings in Energy Conversion and Management.

According to Julian Wang, associate professor of architectural engineering, near-infrared light — a portion of solar light that humans cannot see but can feel as heat — can activate unique light-to-heat effects on certain metallic nanoparticles, enhancing how heat flows inward through a window. 

“We were interested in understanding how these effects could be helpful in saving energy in buildings, particularly during the winter season,” said Wang, who is also affiliated with the College of Art and Architecture’s Department of Architecture and the Materials Research Institute at Penn State. 

The research team first developed a model to estimate how much heat from sunlight would be reflected, absorbed into or transferred through a window coated with metallic nanoparticles. They chose the photothermal compound for its ability to absorb the sun’s near-infrared light, while still allowing for ample visible light transmission. The model predicted that the coating would reflect less near-infrared light or heat, absorbing more of it through the window, than most other coating types. 

Testing a single-pane window coated with the nanoparticles under simulated sunlight in the laboratory, the researchers confirmed predictions made by the simulation. There was a significant rise in temperature on the side of the window coated with the nanoparticles, indicating the coating could pull heat from sunlight inside to compensate for internal heat lost through a single-pane window.

The researchers then implemented their data into a larger-scale simulation to analyze the energy savings for an entire building with coated windows across different climates. Compared to a commercially available single-pane window’s low-emissivity coating, the photothermal coating absorbed much of the light in the near-infrared region of the spectrum while the conventionally coated window reflected it outside. This near-infrared absorption resulted in a roughly 12% to 20% reduction in heat loss compared to the other coatings and an overall building energy saving potential of up to around 20% when compared to a building with no coatings on single-pane windows.

However, better heat transmissivity, a boon in winter, becomes a drawback in the warmer months, Wang said. To account for seasonal changes, the researchers also implemented awnings into their building-scale simulation. This design blocked the more direct rays of sunlight that heat the environment in summer, largely negating the undesirable heat transfer and any associated cooling costs. The team is still investigating other methods, including dynamic window systems, to fulfill seasonal heating and cooling needs.

“As demonstrated by this study, we can still improve the overall thermal performance of single-pane windows to be similar to double-pane windows in the winter season at this research stage,” Wang said. “These findings challenge our conventional solution of using more layers or insulation materials to retrofit single-pane windows for energy savings.”

“Given the significant demands the building stock places on the energy infrastructure, and in turn on the natural environment, it is crucially important that we advance our knowledge to achieve energy-efficient buildings,” said Sez Atamturktur Russcher, Harry and Arlene Schell Professor and Head of the Department of Architectural Engineering. “Dr. Wang and his team are conducting fundamental research that is actionable.”

Other contributors to this work include Enhe Zhang, an architectural engineering doctoral student; Qiuhua Duan, an assistant professor of civil engineering at the University of Alabama who earned her doctorate in architectural engineering at Penn State in December 2021; Yuan Zhao, a research scientist at Advanced NanoTherapies Inc., who contributed to this work as a postdoctoral researcher at Penn State; and Yanxiao Feng, an architectural engineering doctoral candidate. The National Science Foundation and U.S. Department of Agriculture Natural Resources Conservation Service supported this work.

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