The samples of the novel film are shown in both absorbing and reflecting states, confirming that the transmitted visible light changes very little. Image: Youngblood Photonics Lab.
The samples of the novel film are shown in both absorbing and reflecting states, confirming that the transmitted visible light changes very little. Image: Youngblood Photonics Lab.

Homeowners know that the type of windows in a house contribute greatly to heating and cooling efficiency. And that’s a big deal – maintaining indoor temperatures consumes great amounts of energy and accounts for 20–40% of the national energy budgets in developed countries.

New research from the University of Pittsburgh and the University of Oxford in the UK now takes energy efficient windows a step further by proposing a new 'smart window' design that would harvest the Sun’s energy in the winter to warm the house and reflect it in the summer to keep it cool. The research is reported in a paper in ACS Photonics.

“The major innovation is that these windows can change according to seasonal needs,” explained Nathan Youngblood, assistant professor of electrical and computer engineering at the University of Pittsburgh and first author of the paper. “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.”

This is done with a film made up of an optical stack of materials less than 300nm thick, including a very thin active layer made of a chalcogenide-based 'phase-change' material (Ge20Te80). This material can absorb the invisible wavelengths of the Sun’s light and emit it as heat, but it can also be 'switched' so that it reflects those wavelengths of light 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 could even be adjusted so that, for example, 30% of the material is turning away heat while 70% is absorbing and emitting it, allowing for more precise temperature control. In order to create and test their prototypes, the researchers worked with Bodle Technologies, a company that specializes in ultra-thin reflective films that can function as displays by controlling color and light, as well as Eckersley O’Callaghan, a leading engineering and architectural firm, and Plasma App, a thin films company.

“Here, we exploit tuning how invisible wavelengths are transmitted or reflected to modulate temperature,” explained Harish Bhaskaran, professor in the University of Oxford’s Materials Department, who led the research. “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, compared to the double-paned windows typically found in homes, using these windows – including the energy required to control the film – would reduce energy usage by 20–34%.

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

This story is adapted from material from the University of Pittsburgh, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.