Energy-efficient windows could play a key role in reducing the energy needed to heat and cool buildings, which makes up more than one-fifth of all the energy used by developed countries. Today’s efficient windows have two glass panes with low-emissivity coatings that improve insulation by preventing heat transfer. Smart windows that can be electrically triggered to darken and block the sun’s visible and near-IR light are also now on the market. On the verge of ongoing innovation, Researchers at the University of Oxford have collaborated with industry experts to develop an adaptable smart window technology that could reduce the energy usage of an average home by up to a third. 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 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 thermal energy from the sun’s infrared rays is absorbed by the glass and re-emitted as heat – either used to warm the room using transparent electrical heaters in the glass substrate or reflected away to cool the room.
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’, he said.
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 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 thermal and optical properties of current windows are ‘set’ into glass coating as part of the manufacturing process – so the only real adjustment for local climate conditions is the choice of coating. Glass with a higher reflectivity property or a solar control coating lets in noticeably less natural light and vice versa. The researchers estimate that using windows fitted with the new prototype glass – including the energy required to control the film – would save 20 to 34 percent in energy usage.
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.’
Oxford spin-out Bodle Technologies developed the phase change material scaled down for the prototype. ‘This work demonstrates yet another interesting optoelectronic application of phase change materials with the potential to significantly improve our everyday life,’ said Peiman Hosseini, CEO of Bodle Technologies.
Industry collaborators on the project were PlasmaApp and Bodle Technologies who both specialise in thin film displays, and architectural engineers Eckersley O’Callaghan. The work was funded as part of the EPSRC Wearable and Flexible Technologies Collaboration.