A film-coating that allows windows to harvest the sun’s energy in the winter to warm a house and reflect it in the summer to keep it cool is under development in an Anglo-US project.

The research to develop energy efficient windows was undertaken at Oxford University and the University of Pittsburgh and funded as part of the EPSRC Wearable and Flexible Technologies Collaboration. The team’s findings were recently published in ACS Photonics.

“The major 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. “They absorb near infrared light from the sun in the winter and turn it into heat for the inside of a building. In the summer months, the sun can be reflected instead of absorbed.”

The film is said to be made up of an optical stack of materials less than 300nm thick, with a very thin active layer made of phase change materials that absorb invisible wavelengths of the sun’s light and emit it as heat. That same material can be ‘switched’ so that it turns those wavelengths of light away.

“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 for the adoption of green technologies.”

The material could be adjusted for more precise temperature control, such that 30 per cent of the material turns away heat while 70 per cent is absorbing and emitting it.

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