Nanostructures enable smart windows

Researchers have created a new type of smart window that could cut window-cleaning costs, reduce heating bills and boost worker productivity. Developed by University College London (UCL) with support from the Engineering and Physical Sciences Research Council (EPSRC), the window utilises pyramid-like nanostructures engraved onto glass and combined with a thermochromic coating.

At 200 nm in length, the nanostructures are 100 times smaller than a human hair. Controlling the surface morphology at the nanoscale allows scientists to tailor how the glass interacts with liquids and light with high precision. As explained by the leader of the research group, UCL’s Dr Ioannis Papakonstantinou, “The bio-inspired nanostructure amplifies the thermochromics properties of the coating and the net result is a self-cleaning, highly performing smart window.”

The window is ultraresistant to water, so rain hitting the outside forms spherical droplets that roll easily over the surface — picking up dirt, dust and other contaminants and carrying them away. This is due to the pencil-like, conical design of nanostructures engraved onto the glass, trapping air and ensuring only a tiny amount of water comes into contact with the surface. This is different from normal glass, where raindrops cling to the surface, slide down more slowly and leave marks behind.

The glass is coated with a very thin (5–10 nm) film of vanadium dioxide, a cheap and abundant material. During cold periods, the window will stop thermal radiation escaping and so prevent heat loss; during hot periods, it will prevent infrared radiation from the sun entering the building. The UCL team calculates that this could result in a reduction in heating bills of up to 40%.

Finally, the design of the nanostructures also gives the windows the same antireflective properties found in the eyes of moths and other creatures that have evolved to hide from predators. It cuts the amount of light reflected internally in a room to less than 5% — compared with the 20–30% achieved by other prototype vanadium dioxide-coated, energy-saving windows — with this reduction in ‘glare’ providing a big boost to occupant comfort.

A smart glass prototype developed by the UCL team. The dark areas are decorated with the nanostructures which significantly suppress reflections and repel water. Image courtesy of Alaric Taylor, UCL.

Windows made of the glass could be especially well suited to high-rise office buildings. As noted by Dr Papakonstantinou, “Because of the obvious difficulties involved, the cost of cleaning a skyscraper’s windows in its first five years is the same as the original cost of installing them. Our glass could drastically cut this expenditure, quite apart from the appeal of lower energy bills and improved occupant productivity thanks to less glare.”

Discussions are now underway with UK glass manufacturers with a view to driving this new window concept towards commercialisation. The key is to develop ways of scaling up the nanomanufacturing methods that the team has developed to produce the glass, as well as scaling up the vanadium dioxide coating process.

“We also hope to develop a smart film that incorporates our nanostructures and can easily be added to conventional domestic, office, factory and other windows on a DIY basis to deliver the triple benefit of lower energy use, less light reflection and self-cleaning, without significantly affecting aesthetics,” Dr Papakonstantinou said.

Smart windows could begin to reach the market within around 3–5 years, depending on the team’s success in securing industrial interest.

Top image caption: This scanning electron microscope photograph shows the pyramid-like nanostructures engraved onto glass. Image courtesy of Alaric Taylor, UCL.

Source