The new glass has a “spectrally tuneable, low-emissivity” coating that uses a phase change material to control the amount of heat that comes into the room from the window, without affecting the quality of the light.
The glass absorbs thermal energy from the sun’s infrared rays and re-emits it as heat – either used to warm the room using transparent electrical heaters in the glass substrate, or reflected away to cool the room.
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The researchers estimate that using windows fitted with the new prototype glass – including the energy required to control the film – would save 20%-34% in energy usage annually compared with double-glazed windows typically found in homes.
Working as part of the Wearable and Flexible Technologies Collaboration (WAFT), the project team proposed the heat-activated coating that could ‘switch’ in line with heating, cooling, and lighting demands.
The team built a prototype with an active chalcogenide-based phase change material so the glass can adapt to the temperature, to save energy. When it is cold, the infrared rays from the sunlight are harvested and transformed into building heat to cut heating costs. If it is warm, the glass can switch state to reflect the heat and reduce the need for air conditioning.
The active phase change material is adjustable – for example, 30% of the material can turn away heat while 70% is absorbing and emitting it – for more precise temperature control.
This compares with the low-emissivity glass (low-E) glass used in today's double-glazed homes and offices, which is inert. A nano-thin metallic coating reflects the UV and infrared waves in sunlight to reduce heat transfer through the glass, but low-E glass is not as responsive as the new smart window, the University of Oxford claims.
Dr Nathan Youngblood, formerly at Oxford and now at the University of Pittsburgh, said: “These windows can change according to seasonal needs. 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.
“Importantly, visible light is transmitted almost identically in both states, so you wouldn't notice the change in the window. That aesthetic consideration is critical for the adoption of green technologies.”
Harish Bhaskaran, professor at Oxford’s Department of Materials, who led the research and the WAFT consortium, said: “Although significant future research is necessary before this technology can be commercialised, the results show that the concept is very promising and with further research can achieve very good efficiencies.”
The full paper – 'Reconfigurable Low-Emissivity Optical Coating Using Ultrathin Phase Change Materials’ – is available in ACS Photonics.
