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'Smart' window design could keep houses warm in winter and cool in summer – Professional Engineering

Professional Engineering
A new ‘smart’ window design could harvest the Sun’s energy in winter to warm buildings and reflect it in summer to keep them cool, its creators have said.
The researchers, from the University of Pittsburgh, Pennsylvania, and the University of Oxford, said the windows – which can switch between absorbing and reflecting certain wavelengths of light – could reduce annual household energy use by 20-34%.
Maintaining indoor temperatures consumes huge amounts of energy, accounting for 20-40% of the national energy budgets in developed countries.
“The major innovation is that these windows can change according to seasonal needs,” said first author Nathan Youngblood, assistant professor of electrical and computer engineering at Pittsburgh. “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 prototype film is made up of an ‘optical stack’ of materials less than 300 nanometres thick, with a very thin active layer made of phase change materials that can absorb the 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 instead. 
“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 can be adjusted to simultaneously absorb and reflect different levels of the near-infrared rays, allowing for more precise temperature control.
Research leader Harish Bhaskaran from Oxford’s Materials Department said: “Here, we exploit tuning how invisible wavelengths are transmitted or reflected to modulate temperature. These ideas have come to fruition with the aid of our long-standing industrial collaborators, and are the result of long-term research.”
The researchers estimate that using windows based on the technology – including the energy required to control the film – would save 20-34% in household energy use each year, compared to homes using typical double-paned windows.
The work was recently published in the journal ACS Photonics, and was funded as part of the EPSRC Wearable and Flexible Technologies Collaboration.
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