The idea of electrically switchable smart glass has been around for at least a couple of decades. Several different technologies have been developed, but the result has been mostly a handful of niche products.
There have been switchable windows developed for architectural use, but they haven’t really caught on, at least in part because of the cost.
Perhaps the most successful smart technologies for windows are called suspended particle devices. They use nano-scale particles suspended in a liquid and placed between two pieces of glass or plastic, or attached to one layer.
When no voltage is applied, the suspended particles are randomly organized, so they block and absorb light. When voltage is applied, the suspended particles "align" and let light pass. The degree of shading can be varied by varying the voltage.
The technology has been embraced by the automotive and aircraft industries. But what about the architecture-engineering-construction sector?
The industry is facing a growing need to make buildings more energy-efficient in a warming world, as well as dealing with increasing demand for top-flight indoor air quality. This demand is driving research into making better switchable smart glass that might be able to produce significant energy savings.
Now there is word from Netherlands that a scientist there has been able to produce windows that are able to reflect invisible infrared light, but allow visible light through. The result is that energy consumption for cooling and heating buildings has been reduced by as much as 12 per cent.
These new smart windows were developed by Hitesh Khandelwal while he was working toward his PhD at Eindhoven University of Technology. He was awarded his degree last month.
Heating, cooling and lighting buildings account for half the energy consumption in the built environment. Daylight plays an important role. Sunlight coming in the windows causes high indoor temperatures which, in warmer climates, leads to a much more substantial use of air conditioning. Of course, during the winter sunlight can be a source of heating, which saves fuel costs.
Windows that reflect sunlight are not new. But a major disadvantage that many have is that they often reflect visible light and therefore become darker and permanently discoloured. Because the windows are static they will always have a cooling effect, even during the winter when cooling isn’t needed.
The smart reflective windows developed by Khandelwal can keep out light with a wavelength of 700 nm to 1,400 nm. That’s in the infrared range and invisible, but it still contains nearly 50 per cent of the energy from sunlight.
The key thing about these windows is that they can be switched on and off, using an on-off switch or an automatic option based on readings from temperature sensors.
The reflective layer of the window contains organic liquid crystals. If that sounds somewhat esoteric, look at the screen in your smart phone. It contains organic liquid crystals which can selectively reflect light of a certain wavelength if they’re arranged in a special way.
During his research, Khandelwal exploited the same principle. He applied an electrical charge to "align" the crystals to reflect the sunlight. But he found that by adding different liquid crystal molecules, he was able to block nearly all of the sunlight between 700 nm and 1,400 nm. Even with almost all of the invisible light blocked, the transparency of the visible light was still around 90 per cent. That’s comparable with the transparency of most double glazing.
Khandelwal has proven that his cooling works with optimum effect when layered between two panes of glass. But to bring down the costs, he has also developed prototypes of a coating that can be applied on existing panes.
Switchable smart windows are likely to have a future in green construction. They’re just one example of the exciting things happening at the interface where people and technology meet.
Korky Koroluk is an Ottawa-based freelance writer. Send comments to firstname.lastname@example.org.