A group of researchers at Pennsylvania State University investigated the effectiveness of a single-layer window covering that can improve energy savings in the winter. Credit: iStock/@Svetl. All rights reserved.
UNIVERSITY PARK, Pennsylvania — Double-glazed windows sandwiched in with a layer of insulating air can provide greater energy efficiency than single-pane windows, but replacing existing single-pane windows can be costly or technically challenging. A more economical, but less effective option is to cover single-chamber windows with a translucent metal film, which absorbs some of the sun’s heat in winter without compromising the transparency of the glass. To improve coating efficiency, Pennsylvania researchers say nanotechnology can help bring thermal performance up to par with double-glazed windows in winter.
A team from the Pennsylvania Department of Architectural Engineering investigated the energy-saving properties of coatings containing nanoscale components that reduce heat loss and better absorb heat. They also completed the first comprehensive analysis of the energy efficiency of building materials. The researchers published their findings in Energy Conversion and Management.
According to Julian Wang, an associate professor of architectural engineering, near-infrared light — the part of sunlight that humans can’t see but can feel heat — can activate the unique photothermal effect of certain metal nanoparticles, increasing heat flow inward. through the window.
“We’re interested in understanding how these effects can improve the energy efficiency of buildings, especially in winter,” said Wang, who also works at the Institute of Architecture and Materials at the Pennsylvania School of Art and Architecture.
The team first developed a model to estimate how much heat from sunlight would be reflected, absorbed, or transmitted through windows coated with metal nanoparticles. They chose a photothermal compound because of its ability to absorb near-infrared sunlight while still providing sufficient visible light transmission. The model predicts that the coating reflects less near infrared light or heat and absorbs more through the window than most other types of coatings.
The researchers tested single-pane glass windows coated with nanoparticles under simulated sunlight in a lab, confirming simulation predictions. The temperature on one side of the nanoparticle-coated window increased significantly, suggesting that the coating can absorb heat from sunlight from within to compensate for internal heat loss through single-pane windows.
The researchers then fed their data into large-scale simulations to analyze the building’s energy savings under various climate conditions. Compared to the low emissivity coatings of commercially available single windows, photothermal coatings absorb most of the light in the near-infrared spectrum, while traditionally coated windows reflect it outward. This near-infrared absorption results in about 12 to 20 percent less heat loss than other coatings, and the building’s overall energy saving potential reaches about 20 percent compared to uncoated buildings on single-pane windows.
However, Wang said that better thermal conductivity, an advantage in winter, becomes a disadvantage in the warm season. To account for seasonal changes, the researchers also incorporated canopies into their building models. This design blocks the more direct sunlight that heats up the environment in summer, largely eliminating poor heat transfer and any associated cooling costs. The team is still working on other methods, including dynamic window systems to meet seasonal heating and cooling needs.
“As this study shows, at this stage of the study, we can still improve the overall thermal performance of single-glazed windows to be similar to double-glazed windows in winter,” Wang said. “These results challenge our traditional solutions of using more layers or insulation to retrofit single-chamber windows to save energy.”
“Given the huge demand in the building stock for energy infrastructure as well as the environment, it is imperative that we advance our knowledge to create energy efficient buildings,” said Sez Atamtürktur Russcher, Professor Harry and Arlene Schell and Head of Construction Engineering. “Dr. Wang and his team are doing actionable basic research.”
Other contributors to this work include Enhe Zhang, a graduate student in architectural design; Qiuhua Duan, Assistant Professor of Civil Engineering at the University of Alabama, received her PhD in Architectural Engineering from Pennsylvania State University in December 2021; Yuan Zhao, researcher at Advanced NanoTherapies Inc., who contributed to this work as a PhD researcher at Pennsylvania State University, Yangxiao Feng, PhD student in architectural design. The National Science Foundation and the USDA Natural Resources Conservation Service supported this work.
Window coverings (close-up molecules) have been shown to enhance the transfer of heat from outdoor sunlight (orange arrows) to the interior of a building while still providing sufficient light transmission (yellow arrows). Source: Image courtesy of Julian Wang. All rights reserved.