Study: Nanoscale Window Coating Reduces Energy Costs
According to a team of researchers from the Penn State Department of Architectural Engineering, coating windows, particularly single-pane windows, with a translucent metallic film capable of absorbing some solar heat is a more economical option than replacing the windows with double-panes.
Their study has since been published in Energy Conversion and Management.
Window Coatings Research
While double-pane windows are still ultimately more energy efficient than single-pane, or single-panes with a translucent metallic film coating, Penn State researchers believe that with the help of nanotechnology, the coating could help elevate thermal performance to that of double-pane windows in winter.
By examining the energy-saving properties of a coating comprising nanoscale components, the team defined which components would reduce heat loss and better absorb heat. According to Julian Wang, associate professor of architectural engineering, near-infrared light—a portion of solar light that humans feel as heat but cannot see—can activate unique light-to-heat effects on certain metallic nanoparticle. This action thus enhances 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.
“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. These findings challenge our conventional solution of using more layers or insulation materials to retrofit single-pane windows for energy savings.”
For the research, the 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 that incorporated a photothermal compound. The compound was chosen for its ability to absorb the sun’s near-infrared light, while still allowing for ample visible light transmission.
By testing a single-pane window, the researchers were able to confirm that when coated with the nanoparticles under simulated sunlight in the laboratory, more heat was absorbed through the window and reflected less near-infrared light or heat than most other coating types. The researchers also noted that there was a significant rise in temperature on the side of the window coated with the nanoparticles, thus indicating that the coating could pull heat from sunlight inside to compensate for internal heat lost.
After making this discovery, the team then implemented their data into a larger-scale simulation to analyze the energy savings for an entire building with coated windows across different climates. According to their findings, 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, while better heat transmissivity is a vital feature for the winter months, in the summer it could become a drawback. To account for seasonal changes, Wang and her team implemented awnings into their building-scale simulation, which blocked more direct sunlight. While this mitigated the new heat issue, the team is continuing its research into other designs, including dynamic window systems, to fulfill seasonal heating and cooling needs.
“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.
Other Energy-Efficient Coatings Research
In 2020, a new coating was developed at RMIT University (Melbourne, Australia), that researchers said could bring down the cost of energy-saving windows and become a standard in new builds and retrofits. The spray-on coating was described as “ultra-thin, cost-effective and rival[s] the performance of current industry standards for transparent electrodes.”
The process for such a coating relies heavily on raw materials and is made through a time-consuming process, according to RMIT. This new method, however, is fast, scalable and “based on cheaper materials that are readily available.”
At the time, researchers said that the method could both simplify the fabrication of smart windows as well as low-emissivity glass. First author Jaewon Kim, a PhD researcher in Applied Chemistry at RMIT, added that the next steps in the research was developing precursors that would decompose at lower temperatures, allowing the coatings to be deposited on plastics and used in flexible electronics, as well as producing larger prototypes by scaling up the deposition.
In 2018, The Department of Energy’s Lawrence Berkeley National Laboratory, in California, reported that a new type of window coating was in development. At the time, scientists were working on bringing to market what they had dubbed a “thin triple super window,” and noted that the design was at least twice as insulating as 99% of windows on the market.
The popular double-glazed windows consist of two layers of glass, a layer of low-emissivity coating and argon gas in between the glass layers. The lab’s latest innovation inserts a third layer of thin glass in between the two layers of double-glazed windows, adds a second low-e coating and replaces the argon gas with krypton gas, which the lab says is more insulating.
In addition to the new “formula,” the design is the same width and same weight of double-glazed windows, putting it a notch about the triple-glazed windows that are currently available, the lab says, because it avoids having to redesign the window sash and frame.
The lab was also working on making different windows for different climates, and planned to sift through its database of 5,000 glazings and coatings to get optimal selections for each market.
And, in the year prior, a coating company claiming that it could lower home energy costs got a boost from Canada’s federal government. According to news, Kitchener, Ontario-based company 3E Nano Inc. received CA$2.7 million (about $2.15 million) to develop a window coating that the company stated would act more as insulation.
3E Nano cofounder Nicholas Komarnycky told The Record that the company has developed a low-cost energy-control coating that can be used for glass and other transparent media. The coating aimed to block unwanted heat transfer—to keep heat outside in the summer and inside in the winter.
In addition to directly benefitting the consumer with lower energy bills, the heat transfer lockdown also cuts greenhouse gas emissions, which would help Canada achieve its climate goals.
The company added that the coating also has an opportunity for an embedded transparent layer of metal that could be activated to generate heat, an application that could be most useful for car windshields.
The coating can also be fine-tuned for specific wavelengths, extending its service to greenhouses.