‘Liquid Windows’ Could Save Building Energy
Inspired by the skin of certain species of squid, a new multilayered fluidic system developed by researchers at the University of Toronto reportedly has the potential to reduce energy costs in buildings.
“Buildings use a ton of energy to heat, cool and illuminate the spaces inside them,” said Raphael Kay, lead author on the new paper published in the journal PNAS.
“If we can strategically control the amount, type and direction of solar energy that enters our buildings, we can massively reduce the amount of work that we ask heaters, coolers and lights to do.”
The research team also included associate professor Ben Hatton, Ph.D. candidate Charlie Katrycz and assistant professor Alstan Jakubiec.
About the Technology
U of T reports that current “smart” building technologies, like automatic blinds or electrochromic windows, can be used to control the amount of sunlight that enters a room. However, Kay says that these systems are limited in that they cannot differentiate between different wavelengths of light or can control how that light gets distributed spatially.
“Sunlight contains visible light, which impacts the illumination in the building—but it also contains other invisible wavelengths, such as infrared light, which we can think of essentially as heat,” he said.
“In the middle of the day in winter, you’d probably want to let in both—but in the middle of the day in summer, you’d want to let in just the visible light and not the heat. Current systems typically can’t do this—they either block both or neither. They also have no ability to direct or scatter the light in beneficial ways.”
As an alternative, the team developed a platform that optimizes the wavelength, intensity and dispersion of light transmitted through windows using microfluidics. The prototypes reportedly consist of flat sheets of plastic that are permeated with an array of millimeter-thick channels to pump fluids through.
Then, customized pigments, particles or other molecules can be mixed into the fluids to control what kind of light gets through and in what direction the light is distributed. The sheets can also be combined into a multi-layered stack with different optical functions, such as controlling intensity, filtering wavelength or tuning the scattering of transmitted light indoors.
This method can be digitally controlled by pumps to add or remove fluids from each layer, optimizing light transmission within the system.
“It’s simple and low-cost, but it also enables incredible combinatorial control. We can design liquid-state dynamic building facades that do basically anything you’d like to do in terms of their optical properties,” Kay said.
According to the release, the study was inspired by multilayered skin of a squid, which contains stacked layers of specialized organs. This includes chromatophores and iridophores, which control light absorption and impact reflection and iridescence, respectively.
U of T researchers reportedly built detailed computer models that analyzed the potential energy impact of covering a hypothetical building in this type of dynamic façade, informed by physical properties measured from their prototypes. The team also then simulated various control algorithms for activating or deactivating the layers in response to changing ambient conditions.
“If we had just one layer that focuses on modulating the transmission of near-infrared light—so not even touching the visible part of the spectrum—we find that we could save about 25% annually on heating, cooling and lighting energy over a static baseline,” said Kay.
“If we have two layers—infrared and visible—it’s more like 50%. These are very significant savings.”
Hatton noted that the challenge of optimizing them would be an “ideal task” for artificial intelligence, which would be a possible future direction for their research.
“The idea of a building that can learn—that can adjust this dynamic array on its own to optimize for seasonal and daily changes in solar conditions—is very exciting for us,” Hatton said.
“We are also working on how to scale this up effectively so that you could cover a whole building. That will take work but given that this can all be done with simple, non-toxic, low-cost materials, it’s a challenge that can be solved.”
Similar Window Research
In November, a group of researchers published a study on a new transparent window coating that could be utilized to lower the temperature inside buildings. The team set out to design a “transparent radiative cooler” (TRC) using advanced computing technology and artificial intelligence.
In the study’s abstract, researchers described that the TRC was developed on the basis of layered photonic structures using a quantum computing-assisted active learning scheme, which combines active data production, machine learning, and quantum annealing in an iterative loop.
By alternating thin layers of common materials like silicon dioxide, silicon nitride, aluminum oxide or titanium dioxide on a glass base topped with a film of polydimethylsiloxane, researchers said they were able to optimize a coating design that, when fabricated, beat the performance of conventionally designed TRCs.
In addition, the team wrote that the resulting coating design performed better than one of best commercial heat-reduction glasses on the market.
The best-performing TRC developed from the study has the potential to reduce cooling energy consumption by 31% compared with conventional windows, according to the study authors. The coating also has the potential to be utilized in other applications, such as car and truck windows.
At the beginning of 2022, a team of researchers from the Penn State Department of Architectural Engineering found that coating windows, particularly single-pane windows, with a translucent metallic film capable of absorbing some solar heat were a more economical option than replacing the windows with double-panes.
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 the thermal performance to that of double-pane windows in winter.
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.