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% percent in household energy use each year, compared to homes using typical double-paned windows.
The abstract to support this work and research says as follows:
A method for controlling the optical properties of a solid-state film over a broad wavelength range is highly desirable and could have significant commercial impact. One such application is smart glazing technology where near-infrared solar radiation is harvested in the winter and reflected in the summer─an impossibility for materials with fixed thermal and optical properties. Here, we experimentally demonstrate the first spectrally tunable, low-emissivity coating using a chalcogenide-based phase-change material (Ge20Te80), which can modulate the solar heat gain of a window while maintaining neutral-coloration and constant transmission of light at visible wavelengths. We additionally demonstrate the controlled transfer of absorbed near-infrared energy to far-infrared radiation, which can be used to heat a building’s interior and show fast, sub-millisecond switching using transparent electrical heaters integrated on glass substrates. These combined properties result in a smart window that is efficient and aesthetically pleasing – crucial for the successful adoption of green technology.
The work was recently published in the journal ACS Photonics, and was funded as part of the EPSRC Wearable and Flexible Technologies Collaboration.