Film-coating gives windows temperature control

A film coating that allows windows to harvest the sun’s energy in winter to heat a home and reflect it in summer to keep it cool is under development in a British-American project.

The fabricated smart windows are shown in both states, showing that the visible light transmitted changes very little (Image: Youngblood Photonics Lab)

The research into the development of energy-efficient windows was conducted at the University of Oxford and the University of Pittsburgh and was funded as part of the EPSRC Collaboration between wearable and flexible technologies. The team’s findings were recently published in ACS Photonics.

“The most important innovation is that these windows can change according to seasonal needs,” said Nathan Youngblood, assistant professor of electrical and computer engineering at the University of Pittsburgh and first author. “In winter they absorb near-infrared light from the sun and convert it into heat for the inside of a building. In the summer months, the sun can be reflected instead of absorbed.”

The film would consist of an optical stack of materials less than 300 nm thick, with a very thin active layer made of phase-change materials that absorb invisible wavelengths of sunlight and release it as heat. That same material can be ‘switched’ so that it turns those wavelengths of light away.

NTU team develops energy-saving glass

“Importantly, visible light is transmitted almost identically in both states, so you wouldn’t notice the change in the window,” Youngblood said. “That aesthetic consideration is critical to the adoption of green technologies.”

The material can be adjusted for more precise temperature control so that 30 percent of the material dissipates heat while 70 percent absorbs and releases it.

Harish Bhaskaran, professor in Oxford’s materials department who led both the research and the WAFT consortium, said: “Here we use tuning how invisible wavelengths are emitted or reflected to modulate temperature. These ideas came about with the help of our long-standing industrial employees and are the result of lengthy research.”

The researchers estimate that using these windows — including the energy needed to power the foil — would save 20 to 34 percent in energy consumption per year compared to double glazing.

To create and test their prototypes, the researchers collaborated with Bodle Technologies, a company specializing in ultra-thin reflective films that can function as displays by controlling color and light, plus Eckersley O’Callaghan, an engineering and architectural firm, and plasma app, a thin film company.

“This work demonstrates yet another interesting optoelectronic application of phase-change materials with the potential to significantly improve our everyday lives,” said Peiman Hosseini, CEO of Bodle Technologies. “The commercialization of PCM-based tunable low-e glass panels still has a number of significant challenges to overcome; however, these preliminary results prove that the long development road ahead is certainly justified. I believe this technology should be part of any future holistic policy approach to tackle climate change.”

Abhishek Maheswari
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